This combination of three photos released by freethehikers.org shows from left: Joshua Fattal, Shane Bauer, and Sarah Shourd.AP

TEHRAN, Iran — A senior Iranian prosecutor accused three Americans detained on the border with Iraq of espionage on Monday, the first signal that Tehran intends to put them on trial.

The move could set up the Americans — who relatives say were hiking and strayed across the border from Iraq — as potential bargaining chips in Iran’s standoff with the West. The announcement came as Washington and Tehran are maneuvering over a deadlock in negotiations over Iran’s nuclear program.

U.S. Secretary of State Hillary Rodham Clinton called for the release of the three. “We believe strongly that there is no evidence to support any charge whatsoever,” she told reporters in Berlin. “And we would renew our request on behalf of these three young people and their families that the Iranian government exercise compassion and release them, so they can return home.”

Clinton said the U.S. would continue to make that case through the Swiss channels who represent U.S. interests in Tehran.

White House press secretary Robert Gibbs said Monday that the three were hiking near the border with Iraq and are innocent. He called on the Iranian government to release them quickly.

Shane Bauer, Sarah Shourd and Josh Fattal, all graduates of the University of California, Berkeley, were arrested July 31 after straying over the Iranian border from northern Iraq. The U.S. government and their families say there were on a hiking vacation and crossed accidentally.

Tehran chief prosecutor Abbas Jafari Dowlatabadi says the three “have been accused of espionage” and that investigations were continuing, according to the state news agency IRNA. He said an “opinion (on their case) will be given in the not distant future.”

It is not clear from his comments whether formal charges had been made, but such announcements are often a sign that charges are imminent if not already filed. In Iran’s opaque judicial system, the process of indictment and trial often takes place behind closed doors.

In September, Iranian President Mahmoud Ahmadinejad said in an interview with The Associated Press that the Americans had violated the law by crossing the border. “What I can ask is that the judiciary expedites the process and gives it its full attention, and to basically take a look at the case with maximum leniency,” he said.

The timing of the announcement raised the possibility that Iran was using the case to pressure the United States amid the negotiations over its nuclear program. Iran has seemed in the past to use jailed Americans as a bargaining chip.

In January, Iranian-American journalist Roxana Saberi was arrested in Tehran, was convicted of espionage, then released on appeal in May. Two months later, U.S. forces in Iraq freed five Iranians who they had been holding for months.

Iran is also currently holding another American, academic Kian Tajbakhsh, who was arrested amid Iran’s postelection turmoil and was sentenced last month to 12 years in prison for an alleged role in opposition protests.

A Canadian-Iranian reporter for Newsweek, Maziar Bahari, was a defendant in the same mass trial over the protests, which erupted after opposition charges that June presidential elections were fraudulent. Bahari was released on bail last month and left Iran, joining his pregnant wife in London.